Method for charging lithium ion secondary battery and battery charger

ABSTRACT

A lithium ion secondary battery includes a positive electrode including a positive electrode active material layer containing lithium iron phosphate, a negative electrode including a negative electrode active material layer containing graphite, and an electrolyte including a lithium salt and a solvent including ethylene carbonate and diethyl carbonate between the positive electrode and the negative electrode. When the battery temperature of the lithium ion secondary battery or the temperature of an environment in which the lithium ion secondary battery is used is T and given temperatures are T1 and T2 (T1&lt;T2), in the case where T&lt;T1, constant current charge is performed until voltage reaches a given value and then constant voltage charge is performed; in the case where T1≤T&lt;T2, only constant current charge is performed; and in the case where T2≤T, charge is not performed.

BACKGROUND OF THE INVENTION 1. Field of the Invention

An embodiment of the disclosed invention relates to a method forcharging a lithium ion secondary battery and a battery charger.

2. Description of the Related Art

In recent years, with the advance of environmental technology,development of power generation devices (e.g., solar power generationdevices) which pose less burden on the environment than conventionalpower generation methods has been actively conducted. Concurrently withthe development of power generation technology, development of powerstorage devices such as lithium ion secondary batteries, lithium ioncapacitors, and air cells has also been underway.

In particular, demand for a lithium ion secondary battery which is usedas a storage battery for a next-generation clean energy vehicle such asa hybrid electric vehicle (HEV), an electronic vehicle (EV), or aplug-in hybrid electric vehicle (PHEV), or a small consumer product suchas a portable information terminal (e.g., a mobile phone, a smartphone,or a notebook personal computer), a portable music player, or a digitalcamera has rapidly expanded, and the lithium ion secondary batterybecomes an indispensable rechargeable energy supply source.

A lithium ion secondary battery basically has a structure in which anelectrolyte is provided between a positive electrode and a negativeelectrode. Typically, a positive electrode and a negative electrode eachinclude a current collector and an active material provided over thecurrent collector. A material which can occlude and release lithium ionsis used for the active materials of the lithium ion secondary battery(see Patent Document 1).

In the case where graphite is used for a negative electrode activematerial layer and ethylene carbonate is used for a solvent of anelectrolyte of a lithium ion secondary battery, the solvent is reducedand decomposed and a passivating film (also referred to as a solidelectrolyte interface (SEI)) is formed on a surface of the negativeelectrode active material layer. The passivating film prevents theelectrolyte from being further decomposed, and lithium ions can beinserted. In addition, stability of the passivating film influences onsafety of the whole lithium ion secondary battery because thepassivating film protects the negative electrode active material layer(see Patent Document 2 and Non-Patent Document 1).

As a method for charging a lithium ion secondary battery, constantcurrent/constant voltage charge (CCCV charge) in which charge isperformed with constant current and then with constant voltage isdeveloped, which is used as a general method for charging a secondarybattery (see Patent Document 3).

REFERENCE Patent Documents

-   [Patent Document] Japanese Published Patent Application No.    2011-238504-   [Patent Document 2] Japanese Published Patent Application No.    2005-78943-   [Patent Document 3] Japanese Published Patent Application No.    2009-158142

Non-Patent Document [Non-Patent Document 1]

-   Richiuniu Niji Denchi (Lithium Ion Secondary Battery), Zenpachi    Ogumi, Ohmsha, March 2010, pp. 116-124.

SUMMARY OF THE INVENTION

In the case where a lithium ion secondary battery is used for, forexample, a next-generation clean energy vehicle, the lithium ionsecondary battery possibly operates under a high-temperature environmentdepending on an environment in which the next-generation clean energyvehicle is used.

Further, a lithium ion secondary battery itself might have hightemperature due to heat generation in charge and discharge of thelithium ion secondary battery

FIGS. 3A to 3C show measurement results of constant current/constantvoltage charge and constant current discharge. FIG. 3A shows a relationbetween charge capacity and voltage and a relation between dischargecapacity and voltage at an operation temperature of 25° C., FIG. 3Bshows those at an operation temperature of 40° C., and FIG. 3C showsthose at an operation temperature of 60° C. The lithium ion secondarybatteries used in the measurements in FIGS. 3A to 3C were operated inconstant temperature baths and the temperatures of the constanttemperature baths (environment temperatures of the lithium ion secondarybatteries) were the operation temperatures.

In each of the lithium ion secondary batteries used in the measurementsin FIGS. 3A to 3C, aluminum foil was used for a positive electrodecurrent collector. Further, for a positive electrode active material, aconductive additive, and a binding agent included in a positiveelectrode active material layer, lithium iron phosphate (LiFePO₄) coatedwith carbon, acetylene black, and polyvinylidene fluoride (PVDF) wereused, respectively.

Further, copper foil was used for a negative electrode currentcollector. For a negative electrode active material, a conductiveadditive, and a binding agent included in a negative electrode activematerial layer, graphite, acetylene black, and polyvinylidene fluoride(PVDF) were used, respectively.

Further, LiPF₆ was used as a solute of an electrolyte, and ethylenecarbonate (EC) and diethyl carbonate (DEC) were used as solvents of theelectrolyte.

In the charge/discharge measurements shown in FIGS. 3A to 3C, 200 cyclesof constant current charge/discharge at a charge/discharge rate of 1 Cwere performed, and then one cycle of constant current/constant voltagecharge and constant current discharge at a charge/discharge rate of 0.2C was performed to examine capacity at a low rate. After the one cycleof constant current/constant voltage charge and constant currentdischarge at 0.2 C, constant current charge/discharge at acharge/discharge rate of 1 C was performed again.

In the constant current charge/discharge at a charge/discharge rate of 1C, charge was performed with a current of 170 mA/g until voltage reacheda cutoff voltage of 4.3 V, and discharge was performed with a current of170 mA/g until the voltage reached a cutoff voltage of 2 V.

In the constant current/constant voltage charge and the constant currentdischarge at a charge/discharge rate of 0.2 C, charge was performed witha current of 34 mA/g and the charge was continued after voltage reacheda cutoff voltage of 4.3 V, until the current reached a cutoff current of1.7 mA/g. Discharge was performed with a current of 34 mA/g until thevoltage reached a cutoff voltage of 2 V.

FIGS. 3A to 3C each show a relation between capacity and voltage incharge and discharge when one cycle of constant current/constant voltagecharge and constant current discharge at 0.2 C was performed. In thecharge measurement, charge was performed with constant current first.Then, when voltage reached a given value, the voltage was kept at thegiven value (4.3 V in FIGS. 3A to 3C) and charge was performed untilcharge current reached 0.01 C. Discharge was performed with constantcurrent in the discharge measurement.

In the charge measurement in FIGS. 3A to 3C, capacity increases withcharging time; therefore, the horizontal axis can be regarded as an axisindicating time in FIGS. 3A to 3C. In FIGS. 3A to 3C, the voltageincreases while the constant current charge is performed, and thevoltage is constant while the constant voltage charge is performed.

As shown in FIGS. 3A and 3B, at operation temperatures of 25° C. and 40°C., a time range in which constant current charge can be performed iswide and a time range in which constant voltage charge is performed isnarrow. On the other hand, as shown in FIG. 3C, a time range in whichconstant current charge can be performed at an operation temperature of60° C. is narrower than the time ranges at operation temperatures of 25°C. and 40° C. In order to obtain given charge capacity at an operationtemperature of 60° C., constant voltage charge needs to be performedlonger to compensate for the narrow time range in which constant currentcharge can be performed.

Particularly at an operation temperature of 60° C., the time range inwhich constant current charge can be performed becomes narrow and thetime range in which constant voltage charge needs to be performed longerbecause an electrolyte deteriorates. When constant voltage charge isperformed for a long time, charging time at high temperature (anoperation temperature of 60° C.) becomes longer, which might lead tofurther deterioration of the electrolyte.

Thus, as shown in FIGS. 3A to 3C, higher operation temperature leads tonarrowing a time range in which constant current charge can beperformed. In order to obtain given capacity, constant voltage chargeneeds to be performed longer to compensate for the narrow time range inwhich constant current charge is performed. As a result, deteriorationof an electrolyte proceeds. Since a passivating film of an electrode isformed by releasing a lithium ion from a solvate of an electrolyte, thepassivating film also deteriorates or is broken.

Thus, an electrode deteriorates when constant voltage charge at hightemperature is performed, and battery characteristics might becomeworse. However, constant voltage charge needs to be performed longer inorder to obtain given capacity.

FIGS. 4A and 4B show a relation between a cycle number and dischargecapacity when charge/discharge measurements were performed at operationtemperatures of 25° C., 40° C., and 60° C. Note that FIG. 4B is anenlarged view of FIG. 4A around 200 cycles.

In the charge/discharge measurements shown in FIGS. 4A and 4B, similarlyto FIGS. 3A to 3C, 200 cycles of constant current charge/discharge at acharge/discharge rate of 1 C were performed, and then one cycle ofconstant current/constant voltage charge and constant voltage dischargeat 0.2 C was performed in order to examine capacity at a low rate. Afterthe one cycle of constant current/constant voltage charge and constantvoltage discharge at 0.2 C, constant current charge/discharge at acharge/discharge rate of 1 C was performed again.

As shown in FIGS. 4A and 4B, at each of operation temperatures of 25° C.and 40° C. a curve indicating discharge capacity is continuous betweenbefore and after constant current/constant voltage charge at acharge/discharge rate of 0.2 C, while a curve indicating dischargecapacity at 60° C. is discontinuous between before and after constantcurrent/constant voltage charge. In addition, the discharge capacitydrastically drops after constant current/constant voltage charge at 60°C. as compared to the discharge capacity before the charge. In otherwords, it is indicated that irreversible capacity is generated inconstant voltage charge at an operation temperature of 60° C. and thedischarge capacity does not return even when constant current charge isperformed again.

When constant voltage charge is performed at a high temperature of 60°C., a passivating film formed on an electrode (particularly a negativeelectrode) is destroyed. Destroy of the passivating film leads todeterioration of the electrode (particularly the negative electrode). Asa result, a curve indicating discharge capacity is discontinuous.Further, the discharge capacity drastically drops. Thus, constantvoltage charge at high temperature causes deterioration of batterycharacteristics of a lithium ion secondary battery.

In view of the above, an object of an embodiment of the disclosedinvention is to prevent deterioration of an electrode.

Further, an object of an embodiment of the disclosed invention is toprevent deterioration of battery characteristics.

An embodiment of the disclosed invention is a method for charging alithium ion secondary battery including a positive electrode including apositive electrode active material layer containing lithium ironphosphate, a negative electrode including a negative electrode activematerial layer containing graphite, and an electrolyte including alithium salt and a solvent including ethylene carbonate and diethylcarbonate between the positive electrode and the negative electrode.When the battery temperature of the lithium ion secondary battery is Tand given temperatures are T1 and T2 (T1<T2), in the case where T<T1,constant current charge is performed until voltage reaches a given valueand then constant voltage charge is performed; in the case whereT1≤T<T2, only constant current charge is performed; and in the casewhere T2≤T, charge is not performed.

An embodiment of the disclosed invention is a method for charging alithium ion secondary battery including a positive electrode including apositive electrode active material layer containing lithium ironphosphate, a negative electrode including a negative electrode activematerial layer containing graphite, and an electrolyte including alithium salt and a solvent including ethylene carbonate and diethylcarbonate between the positive electrode and the negative electrode.When the temperature of an environment in which the lithium ionsecondary battery is used is T and given temperatures are T1 and T2(T1<T2), in the case where T<T1, constant current charge is performeduntil voltage reaches a given value and then constant voltage charge isperformed; in the case where T1≤T<T2, only constant current charge isperformed; and in the case where T2≤T, charge is not performed.

An embodiment of the disclosed invention is a battery charger includinga power conversion unit for supplying electric power supplied from anelectric power supply portion as constant current or constant voltage; acharge control switch and a discharge control switch each connected tothe power conversion unit in series; a control circuit for controllingoutput of the power conversion unit; and a temperature detection unitfor detecting a battery temperature T of a secondary battery. Thecontrol circuit includes a current-voltage switching unit and atemperature control switching unit. The current-voltage switching unitswitches from constant current charge to constant voltage charge at atime when voltage reaches a given value. The temperature controlswitching unit outputs a signal for supplying output of thecurrent-voltage switching unit to the power conversion unit in the casewhere the battery temperature T detected by the temperature detectionunit is lower than T1, turns off the discharge control switch in thecase where T2≤T, and turns off the charge control switch in the casewhere T1≤T<T2. Note that T1<T2.

An embodiment of the disclosed invention is a battery charger includinga power conversion unit for supplying electric power supplied from anelectric power supply portion as constant current or constant voltage; acharge control switch and a discharge control switch each connected tothe power conversion unit in series; a control circuit for controllingoutput of the power conversion unit; and a temperature detection unitfor detecting a temperature T of an environment in which a secondarybattery is used. The control circuit includes a current-voltageswitching unit and a temperature control switching unit. Thecurrent-voltage switching unit switches from constant current charge toconstant voltage charge at a time when voltage reaches a given value.The temperature control switching unit outputs a signal for supplyingoutput of the current-voltage switching unit to the power conversionunit in the case where the environment temperature T detected by thetemperature detection unit is lower than T1, turns off the dischargecontrol switch in the case where T2≤T, and turns off the charge controlswitch in the case where T1≤T<T2. Note that T1<T2.

In an embodiment of the disclosed invention, the secondary battery is alithium ion secondary battery including a positive electrode including apositive electrode active material layer containing lithium ironphosphate, a negative electrode including a negative electrode activematerial layer containing graphite, and an electrolyte including alithium salt and a solvent including ethylene carbonate and diethylcarbonate between the positive electrode and the negative electrode.

In an embodiment of the disclosed invention, in the case where thebattery temperature or the environment temperature of the lithium ionsecondary battery (temperature T) is higher than or equal to the firsttemperature T1 and lower than the second temperature T2 (T1<T2), chargeis performed with given current (constant current charge); after voltagereaches a given value, constant voltage charge is not performed and theconstant current charge is terminated. Thus, a film formed on anelectrode is prevented from being damaged due to voltage application,leading to prevention of deterioration of the electrode. Further,deterioration of battery characteristics of the lithium ion secondarybattery can be prevented.

In the case where the battery temperature or the environment temperatureof the lithium ion secondary battery (temperature T) is lower than thefirst temperature T1, constant current charge is performed until voltagereaches a given value and then constant voltage charge is performedafter the voltage reaches the given value. Thus, capacity of the lithiumion secondary battery can be increased.

In the case where the battery temperature or the environment temperatureof the lithium ion secondary battery (temperature T) is higher than orequal to the second temperature T2, charge is not performed because anelectrode might deteriorate even in constant current charge.

In an embodiment of the disclosed invention, the temperature detectionunit is a thermistor.

In an embodiment of the disclosed invention, the temperature T1 ishigher than 40° C. and lower than or equal to 60° C.

In an embodiment of the disclosed invention, the temperature T2 ishigher than 60° C.

According to an embodiment of the disclosed invention, deterioration ofan electrode can be prevented.

According to an embodiment of the disclosed invention, deterioration ofbattery characteristics can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a cross-sectional view of a lithium ion secondary battery:

FIG. 2 is a flow chart showing a charge method;

FIGS. 3A to 3C each show a relation between capacity and voltage;

FIGS. 4A and 4B show a relation between a cycle number and dischargecapacity;

FIGS. 5A and 5B are a top view and a cross-sectional view of a lithiumion secondary battery;

FIG. 6 illustrates examples of electric devices;

FIGS. 7A to 7C illustrate an example of an electric device;

FIGS. 8A and 8B illustrate an example of an electric device;

FIGS. 9A to 9D are top views and cross-sectional views of graphene;

FIG. 10 is a circuit diagram of a battery charger; and

FIG. 11 is a circuit diagram of a battery charger.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the invention disclosed in this specification will behereinafter described with reference to the accompanying drawings. Notethat the invention disclosed in this specification can be carried out ina variety of different modes, and it is easily understood by thoseskilled in the art that the modes and details of the invention disclosedin this specification can be changed in various ways without departingfrom the spirit and scope thereof. Therefore, the present invention isnot construed as being limited to description of the embodiment. Notethat, in the drawings hereinafter shown, the same portions or portionshaving similar functions are denoted by the same reference numerals, anddescription thereof will be omitted. Further, in some cases, the samehatching patterns are applied to similar parts, and the similar partsare not necessarily designated by reference numerals.

Note that the position, size, range, or the like of each structure shownin the drawings and the like is not accurately represented in some casesfor easy understanding. Therefore, the disclosed invention is notnecessarily limited to the position, size, range, or the like asdisclosed in the drawings and the like.

In this specification and the like, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not mean limitation of the number ofcomponents.

<Structure of Lithium Ion Secondary Battery>

In a lithium ion secondary battery 130 illustrated in FIG. 1, a positiveelectrode 102, a negative electrode 107, and a separator 110 areprovided in a housing 120 which isolates the components from theoutside, and the housing 120 is filled with an electrolyte 111. Theseparator 110 is provided between the positive electrode 102 and thenegative electrode 107.

In the positive electrode 102, a positive electrode active materiallayer 101 is provided in contact with a positive electrode currentcollector 100. In this specification, the positive electrode activematerial layer 101 and the positive electrode current collector 100provided with the positive electrode active material layer 101 arecollectively referred to as the positive electrode 102.

On the other hand, a negative electrode active material layer 106 isprovided in contact with a negative electrode current collector 105. Inthis specification, the negative electrode active material layer 106 andthe negative electrode current collector 105 provided with the negativeelectrode active material layer 106 are collectively referred to as thenegative electrode 107.

The positive electrode current collector 100 and the negative electrodecurrent collector 105 are connected to a terminal portion 121 and aterminal portion 122, respectively. Charge and discharge are performedthrough the terminal portion 121 and the terminal portion 122.

Although, in the illustrated structure, there are gaps between thepositive electrode active material layer 101 and the separator 110 andbetween the negative electrode active material layer 106 and theseparator 110, an embodiment of the present invention is not limited tothis structure. The positive electrode active material layer 101 may bein contact with the separator 110, and the negative electrode activematerial layer 106 may be in contact with the separator 110. Further,the lithium ion secondary battery 130 may be rolled into a cylinder withthe separator 110 provided between the positive electrode 102 and thenegative electrode 107.

The positive electrode current collector 100 can be formed using ahighly conductive material such as a metal typified by stainless steel,gold, platinum, zinc, iron, copper, aluminum, or titanium, or an alloythereof. Alternatively, the positive electrode current collector 100 canbe formed using an aluminum alloy to which an element which improvesheat resistance, such as silicon, titanium, neodymium, scandium, ormolybdenum, is added. Further alternatively, the positive electrodecurrent collector 100 may be formed using a metal element which formssilicide by reacting with silicon. Examples of the metal element whichforms silicide by reacting with silicon include zirconium, titanium,hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten,cobalt, nickel, and the like. The positive electrode current collector100 can have a foil-like shape, a plate-like shape (a sheet-like shape),a net-like shape, a punching-metal shape, an expanded-metal shape, orthe like as appropriate. In this embodiment, aluminum foil is used asthe positive electrode current collector 100.

In this embodiment, lithium iron phosphate (LiFePO₄) having an olivinestructure is used as a positive electrode active material included inthe positive electrode active material layer 101.

In lithium iron phosphate having an olivine structure, the diffusionpath of lithium ions is unidimensional. Thus, as crystallinity is high,the diffusion path of lithium ions is ensured and insertion andextraction of a large amount of lithium ions is possible. Further, sincelithium iron phosphate includes iron, the capacitance is large. Inaddition, iron phosphate (FePO₄) which is obtained by completelyextracting lithium from lithium iron phosphate is also stable;therefore, the capacity of a lithium ion secondary battery formed usinglithium iron phosphate can be increased safely.

Note that an active material refers to a material that relates tointercalation and deintercalation of ions which function as carriers.When an electrode (a positive electrode, a negative electrode, or bothof them) is formed, an active material layer in which an active materialis mixed with a conductive additive, a binding agent, a solvent, and thelike is formed over a current collector. Thus, the active material andthe active material layer are distinguished. Accordingly, the positiveelectrode active material and the positive electrode active materiallayer 101 are distinguished and a negative electrode active material tobe described later and the negative electrode active material layer 106are distinguished.

The positive electrode active material layer 101 may include a knownconductive additive or binding agent (also referred to as a binder). Inthis embodiment, acetylene black (AB) is used as a conductive additiveand polyvinylidene fluoride (PVDF) is used as a binding agent.

The negative electrode current collector 105 is formed using a highlyconductive material such as metal, for example. As the highly conductivematerial, stainless steel, iron, aluminum, copper, nickel, or titaniumcan be used, for example. The negative electrode current collector 105can have a foil-like shape, a plate-like shape (a sheet-like shape), anet-like shape, a punching-metal shape, an expanded-metal shape, or thelike as appropriate. In this embodiment, copper foil is used as thenegative electrode current collector 105.

The negative electrode active material layer 106 includes a negativeelectrode active material which can occlude and release ions serving ascarriers. In this embodiment, spherical graphite (grain size of 9 μm) isused as the negative electrode active material included in the negativeelectrode active material layer 106.

A passivating film formed by reduction and decomposition of ethylenecarbonate (EC) serving as a solvent (to be described later) of theelectrolyte 111 is formed on a surface of the graphite used as thenegative electrode active material. With the passivating film, thesolvent is prevented from further being decomposed and intercalation oflithium ions into the graphite which is the negative electrode activematerial is possible.

The negative electrode active material layer 106 may include a knownconductive additive or binding agent. In this embodiment, acetyleneblack (AB) is used as a conductive additive and polyvinylidene fluoride(PVDF) is used as a binding agent.

The negative electrode active material layer 106 may be predoped withlithium. Predoping with lithium may be performed in such a manner that alithium layer is formed on a surface of the negative electrode activematerial layer 106 by a sputtering method. Alternatively, lithium foilis provided on the surface of the negative electrode active materiallayer 106, whereby the negative electrode active material layer 106 canbe predoped with lithium.

The electrolyte 111 includes a solute and a solvent. As the solute ofthe electrolyte 111, a material including carrier ions is used. Typicalexamples of the solute include lithium salts such as LiPF₆, LiClO₄,LiAsF₆, LiBF₄, and Li(C₂F₅SO₂)₂N. In this embodiment, LiPF₆ is used asthe solute.

As the solvent of the electrolyte 111, a material in which carrier ionscan transfer is used. As the solvent of the electrolyte, an aproticorganic solvent is preferably used. In this embodiment, a mixed solutionof ethylene carbonate (EC) and diethyl carbonate (DEC) is used.

As described above, ethylene carbonate is reduced and decomposed and apassivating film is formed on a surface of the graphite which is thenegative electrode active material; therefore, ethylene carbonate issuitable for the solvent of the electrolyte 111. However, since ethylenecarbonate is in a solid state at room temperature, a solution in whichethylene carbonate is dissolved in diethyl carbonate is used as thesolvent.

An insulating porous material can be used as the separator 110. Forexample, paper; nonwoven fabric; a glass fiber; ceramics; a syntheticfiber containing nylon (polyamide), vinylon (polyvinyl alcohol basedfiber), polyester, acrylic, polyolefin, or polyurethane; or the like maybe used. Note that a material which is not dissolved in the electrolyte111 should be selected.

A structure in which a positive electrode active material is coveredwith graphene may be employed. Graphene refers to a sheet of one atomiclayer of carbon molecules having sp² bonds. Graphene includessingle-layer graphene and multilayer graphene.

Note that graphene in this specification includes single-layer grapheneand multilayer graphene including two to hundred layers. Single-layergraphene refers to a sheet of one atomic layer of carbon moleculeshaving π bonds. Graphene oxide refers to a compound formed by oxidationof such graphene. When graphene oxide is reduced to form graphene,oxygen contained in the graphene oxide is not entirely released and partof the oxygen remains in the graphene. When the graphene containsoxygen, the proportion of oxygen is higher than or equal to 2 atomic %and lower than or equal to 20 atomic %, preferably higher than or equalto 3 atomic % and lower than or equal to 15 atomic %.

Graphene is chemically stable and has favorable electriccharacteristics. Graphene has high conductivity because six-memberedrings each composed of carbon atoms are connected in the planardirection. That is, graphene has high conductivity in the planardirection. Graphene has a sheet-like shape and a gap is provided betweenstacked graphene layers in a direction parallel to the plane, so thations can transfer in the gap. However, the transfer of ions in thedirection perpendicular to the graphene layers is difficult.

When a surface of the active material is in contact with an electrolytein the lithium ion secondary battery as described above, the electrolyteand the active material react with each other, so that a film is formedon the surface of the active material. The film is called a solidelectrolyte interface (SEI) which is considered necessary for relievingreaction between the active material and the electrolyte and forstabilization. However, when the thickness of the film is increased,carrier ions are less likely to be occluded in an electrode, leading toproblems such as a reduction in conductivity of carrier ions between theactive material and the electrolyte and a waste of the electrolyte.Thus, the positive electrode active material is covered with graphene,whereby an increase in thickness of the film can be suppressed. As aresult, a reduction in conductivity of carrier ions and a waste of theelectrolyte can be suppressed.

FIG. 9A is a top view of the positive electrode active material layer101. The positive electrode active material layer 101 contains positiveelectrode active materials 117 which are particles capable of occludingand releasing carrier ions, and graphenes 118 which cover a plurality ofpositive electrode active materials 117 and at least partly surround theplurality of positive electrode active materials 117. A plurality ofgraphenes 118 covers surfaces of the plurality of positive electrodeactive materials 117. The positive electrode active materials 117 maypartly be exposed.

The size of the particle of the positive electrode active material 117is preferably greater than or equal to 20 nm and less than or equal to100 nm. Note that the size of the particle of the positive electrodeactive material 117 is preferably smaller because electrons transfer inthe positive electrode active materials 117.

In addition, sufficient characteristics can be obtained even whensurfaces of the positive electrode active materials 117 are not coveredwith graphene; however, it is preferable to use both the graphene andthe positive electrode active material because carriers transfer hoppingbetween the positive electrode active materials and current flows.

FIG. 9B is a cross-sectional view of part of the positive electrodeactive material layer 101 in FIG. 9A. The positive electrode activematerial layer 101 contains the positive electrode active materials 117and the graphenes 118 covering the positive electrode active materials117. The graphenes 118 are observed to have linear shapes in crosssection. The plurality of positive electrode active materials 117 issurrounded with one graphene or plural graphenes. That is, the pluralityof particles of the positive electrode active materials exists withinone graphene or among plural graphenes. Note that the graphene has abag-like shape, and the plurality of particles of the positive electrodeactive materials is surrounded with the bag-like portion in some cases.In addition, the positive electrode active materials are not coveredwith the graphenes and partly exposed in some cases.

Note that the positive electrode active material layer 101 may include aknown binding agent such as acetylene black particles or carbonparticles having a one-dimensional expansion (e.g., carbon nanofibers),which have a volume 0.1 times to 10 times as large as that of thegraphene.

As an example of the positive electrode active material 117, a materialwhose volume expands by occlusion of ions serving as carriers is given.When such a material is used, the positive electrode active materiallayer 101 becomes friable and is partly broken by charge and discharge,resulting in lower reliability of a lithium ion secondary battery.However, even when the volume of the positive electrode active materialexpands due to charge and discharge, the graphene partly covers theperiphery of the positive electrode active material, which can preventdispersion of the positive electrode active material and the break ofthe positive electrode active material layer 101. That is to say, thegraphene has a function of maintaining the bond between the positiveelectrode active materials even when the volume of the positiveelectrode active materials is increased and de-creased by charge anddischarge. Thus, a highly reliable lithium ion secondary battery can bemanufactured with graphene used for the positive electrode activematerial layer 101.

The graphene 118 is in contact with the plurality of particles of thepositive electrode active materials and also serves as a conductiveadditive. Further, the graphene 118 has a function of holding thepositive electrode active materials 117 capable of occluding andreleasing carrier ions. Thus, a binding agent does not need to be mixedinto the positive electrode active material layer. Accordingly, theproportion of the positive electrode active materials in the positiveelectrode active material layer can be increased, which allows anincrease in discharge capacity of a lithium ion secondary battery.

Similarly to the positive electrode active material, the negativeelectrode active material may be covered with graphene. FIG. 9C is a topview of part of the negative electrode active material layer 106. Thenegative electrode active material layer 106 contains negative electrodeactive materials 132, which are particles, and graphenes 133 which covera plurality of particles of the negative electrode active materials 132.In the planar view of the negative electrode active material layer 106,the different graphenes 133 cover surfaces of the plurality of particlesof the negative electrode active materials 132. The negative electrodeactive materials 132 may partly be exposed.

FIG. 9D is a cross-sectional view of part of the negative electrodeactive material layer 106 in FIG. 9C. FIG. 9D illustrates the negativeelectrode active materials 132 and the graphenes 133. The graphenes 133cover the negative electrode active materials 132 in the negativeelectrode active material layer 106 in the plan view. The graphenes 133are observed to have linear shapes in cross section. One graphene orplural graphenes overlap with the plurality of negative electrode activematerials 132, or the plurality of particles of the negative electrodeactive materials 132 is at least partly surrounded with one graphene orplural graphenes. Note that the graphene 133 has a bag-like shape, andthe plurality of particles of the negative electrode active material issurrounded with the bag-like portion in some cases. The graphene 133partly has openings where the negative electrode active materials 132are exposed in some cases.

The desired thickness of the negative electrode active material layer106 is determined in the range of 20 μm to 100 μm.

The negative electrode active material layer 106 may contain a knownbinding agent such as polyvinylidene fluoride, and a known conductiveadditive such as acetylene black particles or carbon particles having aone-dimensional expansion (e.g., carbon nanofibers), which have a volume0.1 times to 10 times as large as that of the graphene.

The negative electrode active material layer 106 may be predoped withlithium. Predoping with lithium may be performed in such a manner that alithium layer is formed on a surface of the negative electrode activematerial layer 106 by a sputtering method. Alternatively, lithium foilis provided on the surface of the negative electrode active materiallayer 106, whereby the negative electrode active material layer 106 canbe predoped with lithium. Particularly in the case where the graphene118 is formed in the positive electrode active material layer 101 aftera lithium ion secondary battery is completed, the negative electrodeactive material layer 106 is preferably predoped with lithium.

As an example of the negative electrode active material 132, a materialwhose volume is expanded by occlusion of carrier ions is given. Whensuch a material is used, the negative electrode active material layer106 becomes friable and is partly broken by charge and discharge,resulting in lower reliability (e.g., inferior cycle characteristics) ofa lithium ion secondary battery. However, the graphene 133 covering theperiphery of the negative electrode active materials 132 in the negativeelectrode 107 in the lithium ion secondary battery of this embodimentcan prevent the negative electrode active materials 132 from beingpulverized and can prevent the negative electrode active material layer106 from being broken, even when the volume of the negative electrodeactive materials 132 is increased and decreased due to charge anddischarge. That is to say, the graphene 133 included in the negativeelectrode 107 in the lithium ion secondary battery of this embodimenthas a function of maintaining the bond between the negative electrodeactive materials 132 even when the volume of the negative electrodeactive materials 132 is increased and decreased due to charge anddischarge. Accordingly, durability of the lithium ion secondary batterycan be improved with the use of graphene for the negative electrodeactive material layer 106.

Thus, a binding agent does not need to be used in forming the negativeelectrode active material layer 106. Accordingly, the proportion of thenegative electrode active materials in the negative electrode activematerial layer with certain weight (certain volume) can be increased,leading to an increase in charge and discharge capacity per unit weight(unit volume) of the electrode.

The graphene 133 has conductivity and is in contact with a plurality ofparticles of the negative electrode active materials 132; thus, it alsoserves as a conductive additive. Thus, a conductive additive does notneed to be used in forming the negative electrode active material layer106. Accordingly, the proportion of the negative electrode activematerials in the negative electrode active material layer with certainweight (certain volume) can be increased, leading to an increase incharge and discharge capacity per unit weight (unit volume) of theelectrode.

With the use of the graphene 133, a sufficient conductive path(conductive path of carrier ions) is formed efficiently in the negativeelectrode active material layer 106, so that the negative electrodeactive material layer 106 and the negative electrode 107 have highconductivity. Accordingly, the capacity of the negative electrode activematerial 132 in the lithium ion secondary battery including the negativeelectrode 107, which is almost equivalent to the theoretical capacity,can be utilized efficiently; thus, the charge capacity can besufficiently high.

Note that the graphene 133 also functions as a negative electrode activematerial capable of occluding and releasing carrier ions, leading to anincrease in charge capacity of the negative electrode 107.

<Method for Charging Lithium Ion Secondary Battery>

FIG. 2 is a flow chart showing a method for charging a lithium ionsecondary battery of this embodiment.

After start of charge (S101), a temperature detection element detects abattery temperature of a lithium ion secondary battery or a temperatureof an environment in which the lithium ion secondary battery is used (Si102). In the case where the detected temperature is higher than or equalto a given second temperature T2 (S103), charge is terminated (S108).

Note that the second temperature T2 is higher than a first temperatureT1 to be described later (T2>T1). In this embodiment, the secondtemperature T2 is higher than 60° C., for example, 90° C., and the firsttemperature T1 is higher than 40° C. and lower than or equal to 60° C.,for example.

In the case where the battery temperature of the lithium ion secondarybattery or the temperature of the environment in which the lithium ionsecondary battery is used is higher than or equal to the secondtemperature T2, the lithium ion secondary battery is not charged becausethe lithium ion secondary battery might deteriorate even in constantcurrent charge.

In the case where the temperature detected in the step S102 is lowerthan the second temperature T2, constant current charge is performed(S104). The constant current charge proceeds and when voltage reaches agiven value (S105), the temperature detection element detects thebattery temperature or the environment temperature (S106).

In the case where the temperature detected in the step S106 is lowerthan the first temperature T1 (S107), constant current charge is changedto constant voltage charge, and the constant voltage charge is performed(S111). The constant voltage charge is performed for a given time, andthen the charge is terminated (S112).

In the case where the temperature detected in the step S106 is higherthan or equal to the first temperature T1 (S107), constant voltagecharge is not performed and the constant current charge is terminated(S112).

As described above, in the charging method of this embodiment, in thecase where the battery temperature or the environment temperature of thelithium ion secondary battery is lower than the second temperature,first, constant current charge is performed until voltage reaches agiven value. Then, after the voltage reaches the given value, thebattery temperature or the environment temperature of the lithium ionsecondary battery is detected. In the case where the temperature islower than the first temperature, constant voltage charge is performed.In the case where the temperature is higher than or equal to the firsttemperature, constant voltage charge is not performed and the charge ofthe lithium ion secondary battery is terminated. Thus, a film formed onan electrode is not destroyed and deterioration of the electrode can beprevented.

In addition, deterioration of battery characteristics can be preventedby the charging method of this embodiment.

<Battery Charger>

FIG. 10 and FIG. 11 each illustrate a circuit diagram of a batterycharger of this embodiment.

<<Structure of Battery Charger>>

A battery charger 200 illustrated in FIG. 10 includes a resistor 202, acharge control switch 205, a discharge control switch 208, a powerconversion circuit 215, an electric power supply portion 217, a negativetemperature coefficient (NTC) thermistor 221 serving as a temperaturedetection element, and a control circuit 222. A secondary battery 201and a load 209 are electrically connected to the battery charger 200 inFIG. 10.

A positive electrode of the secondary battery 201 is electricallyconnected to one terminal of the resistor 202 and a terminal CSIN of thecontrol circuit 222. A negative electrode of the secondary battery 201is grounded. Note that the voltage value of the secondary battery 201 isequal to the value of voltage applied to the terminal CSIN of thecontrol circuit 222. The aforementioned lithium ion secondary battery130 may be used as the secondary battery 201.

Here, the phrase “being electrically connected” includes the case ofbeing electrically connected indirectly as well as the case of beingelectrically connected directly. Therefore, for example, the positiveelectrode of the secondary battery 201 may be directly electricallyconnected to the one terminal of the resistor 202 and the terminal CSINof the control circuit 222 or may be electrically connected to the oneterminal of the resistor 202 and the terminal CSIN of the controlcircuit 222 through another electrode or wiring.

Although the NTC thermistor 221 is used as the temperature detectionelement in FIG. 10, the temperature detection element is not limitedthereto. Any element capable of detecting the battery temperature or theenvironment temperature of the secondary battery 201 can be used. Athermistor is a resistor whose electrical resistance greatly changeswith temperature. An NTC thermistor is a thermistor whose resistancedecreases with temperature rise. The NTC thermistor 221 illustrated inFIG. 10 is provided in the vicinity of the secondary battery 201 anddetects the temperature of the environment in which the secondarybattery 201 is used.

One terminal of the NTC thermistor 221 in FIG. 10 is electricallyconnected to a terminal THM of the control circuit 222. The otherterminal of the NTC thermistor 221 is grounded.

The resistor 202 detects current flowing through the secondary battery201. The one terminal of the resistor 202 is electrically connected tothe positive electrode of the secondary battery 201 and the terminalCSIN of the control circuit 222. The other terminal of the resistor 202is electrically connected to a first terminal of the charge controlswitch 205 and a terminal CSIP of the control circuit 222.

The resistance of the resistor 202 is predetermined and voltage betweenthe terminal CSIN and the terminal CSIP of the control circuit 222 isapplied to the resistor 202. The value of current flowing through theresistor 202 is determined by the resistance of the resistor 202 and thevoltage applied to the resistor 202.

The charge control switch 205 includes a diode 203 and an n-channeltransistor 204. An input terminal of the diode 203 is the first terminalof the charge control switch 205 and is electrically connected to one ofa source and a drain of the n-channel transistor 204. An output terminalof the diode 203 is a second terminal of the charge control switch 205and is electrically connected to the other of the source and the drainof the n-channel transistor 204 and a first terminal of the dischargecontrol switch 208.

The one of the source and the drain of the n-channel transistor 204 isthe first terminal of the charge control switch 205 and is electricallyconnected to the input terminal of the diode 203. The other of thesource and the drain of the n-channel transistor 204 is the secondterminal of the charge control switch 205 and is electrically connectedto the output terminal of the diode 203 and the first terminal of thedischarge control switch 208. A gate of the n-channel transistor 204 isa third terminal of the charge control switch 205 and is electricallyconnected to a terminal CHA of the control circuit 222.

The charge control switch 205 is an element for automaticallyterminating charge of the secondary battery 201, i.e., a limiter. Then-channel transistor 204 is in an on state in a normal time while then-channel transistor 204 is in an off state in an emergency time (timewhen charge is automatically terminated). The diode 203 does not preventdischarge current from flowing from the secondary battery 201.

The discharge control switch 208 includes a diode 206 and an n-channeltransistor 207. An output terminal of the diode 206 is the firstterminal of the discharge control switch 208 and is electricallyconnected to the second terminal of the charge control switch 205 andone of a source and a drain of the n-channel transistor 207. An inputterminal of the diode 206 is a second terminal of the discharge controlswitch 208 and is electrically connected to the load 209, a firstterminal of the power conversion circuit 215, and the other of thesource and the drain of the n-channel transistor 207.

The one of the source and the drain of the n-channel transistor 207 isthe first terminal of the discharge control switch 208 and iselectrically connected to the second terminal of the charge controlswitch 205 and the output terminal of the diode 206. The other of thesource and the drain of the n-channel transistor 207 is the secondterminal of the discharge control switch 208 and is electricallyconnected to the load 209, the first terminal of the power conversioncircuit 215, and the input terminal of the diode 206. A gate of then-channel transistor 207 is a third terminal of the discharge controlswitch 208 and is electrically connected to a terminal DIS of thecontrol circuit 222.

The discharge control switch 208 is an element for automaticallyterminating discharge of the secondary battery 201, i.e., a limiter. Then-channel transistor 207 is in an on state in a normal time while then-channel transistor 207 is in an off state in an emergency time (timewhen discharge is automatically terminated). The diode 206 does notprevent charge current from flowing into the secondary battery 201.

The load 209 is electrically connected to the second terminal of thedischarge control switch 208 and the first terminal of the powerconversion circuit 215. Depending on the potentials of the secondterminal of the discharge control switch 208 and the first terminal ofthe power conversion circuit 215 both electrically connected to the load209, the secondary battery 201 is charged by the power conversioncircuit 215 or discharges electricity to the load 209.

The power conversion circuit 215 has a function of converting electricpower supplied from a direct-current power source 216 to be describedlater into constant current (in the case of constant current charge) orconstant voltage (in the case of constant voltage charge) and supplyingwhen the secondary battery 201 is charged by the direct-current powersource 216. The power conversion circuit 215 includes a coil 211, adiode 212, and an n-channel transistor 213. One terminal of the coil 211is the first terminal of the power conversion circuit 215. The otherterminal of the coil 211 is electrically connected to an output terminalof the diode 212 and one of a source and a drain of the n-channeltransistor 213.

The output terminal of the diode 212 is electrically connected to theother terminal of the coil 211 and the one of the source and the drainof the n-channel transistor 213. An input terminal of the diode 212 isgrounded.

The one of the source and the drain of the n-channel transistor 213 iselectrically connected to the other terminal of the coil 211 and theoutput terminal of the diode 212. The other of the source and the drainof the n-channel transistor 213 is a second terminal of the powerconversion circuit 215. A gate of the n-channel transistor 213 is athird terminal of the power conversion circuit 215 and is electricallyconnected to a terminal GS of the control circuit 222.

The electric power supply portion 217 supplies electric power forcharging the secondary battery 201. Although the direct-current powersource 216 is used for the electric power supply portion 217 in FIG. 10,the electric power supply portion 217 is not limited thereto. Theelectric power supply portion 217 may have a structure in which analternate current-direct current converter is included, alternatecurrent power is supplied from an external alternate-current powersource, and the alternate current power is converted into direct currentpower by the alternate current-direct current converter. Thedirect-current power source 216, the alternate current-direct currentconverter, the alternate-current power source, or the like may beincorporated or externally provided. A positive electrode of thedirect-current power source 216 in FIG. 10 is electrically connected tothe second terminal of the power conversion circuit 215. A negativeelectrode of the direct-current power source 216 is grounded.

FIG. 11 illustrates a detailed structure of the control circuit 222. Thecontrol circuit 222 includes a current control circuit 235, a voltagecontrol circuit 244, a current-voltage control switch circuit 252, atemperature control switch circuit 276, a multiplexer 281, an ANDcircuit 282, a level shifter 283, a NAND circuit 284, a level shifter285, and a level shifter 286.

The current control circuit 235 controls a potential output from theterminal GS of the control circuit 222 in constant current charge. Thepotential output from the terminal GS of the control circuit 222 isapplied to the gate of the n-channel transistor 213 of the powerconversion circuit 215. The n-channel transistor 213 is switched on andoff in accordance with the potential applied to the gate of then-channel transistor 213, so that charge of the secondary battery 201 bythe direct-current power source 216 is controlled. The current controlcircuit 235 includes an instrumentation amplifier 231, a comparator 232,a flip-flop 233, and an oscillation circuit 234.

The instrumentation amplifier 231 is an element which amplifies adifference between voltage input to a non-inverting input terminal andvoltage input to an inverting input terminal by K times to output. Thenon-inverting input terminal of the instrumentation amplifier 231 is theterminal CSIP of the control circuit 222 and is a first terminal of thecurrent control circuit 235. The inverting input terminal of theinstrumentation amplifier 231 is the terminal CSIN of the controlcircuit 222 and is a second terminal of the current control circuit 235.Note that a third terminal of the current control circuit 235 branchesoff from the second terminal of the current control circuit 235 and iselectrically connected to a first terminal of the voltage controlcircuit 244. An output terminal of the instrumentation amplifier 231 iselectrically connected to a non-inverting input terminal of thecomparator 232.

The comparator 232 is an element which compares voltage input to anon-inverting input terminal with voltage input to an inverting inputterminal to change output depending on which voltage is higher. Thenon-inverting input terminal of the comparator 232 is electricallyconnected to the output terminal of the instrumentation amplifier 231.First reference voltage Vref1 is input to an inverting input terminal ofthe comparator 232. An output terminal of the comparator 232 iselectrically connected to an input terminal R of the flip-flop 233.

An RS flip-flop is used as the flip-flop 233 illustrated in FIG. 11. Theinput terminal R of the flip-flop 233 is electrically connected to theoutput terminal of the comparator 232. A pulse signal generated by theoscillation circuit 234 is input to an input terminal S of the flip-flop233. An output terminal Q of the flip-flop 233 is a fourth terminal ofthe current control circuit 235 and is electrically connected to aninput terminal A of the multiplexer 281.

The oscillation circuit 234 generates a pulse signal with a short on/offcycle, i.e., a pulse signal with a short duty cycle The pulse signalgenerated by the oscillation circuit 234 is input to the input terminalS of the flip-flop 233.

The voltage control circuit 244 controls a potential output from theterminal GS of the control circuit 222 in constant voltage charge. Thepotential output from the terminal GS of the control circuit 222 isapplied to the gate of the n-channel transistor 213 of the powerconversion circuit 215. The n-channel transistor 213 is switched on andoff in accordance with a potential applied to the gate of the n-channeltransistor 213, so that charge of the secondary battery 201 by thedirect current power source 216 is controlled. The voltage controlcircuit 244 includes an error amplifier 241, a comparator 242, and atriangle wave oscillation circuit 243.

The error amplifier 241 (also referred to as an integrator) amplifies adifference between voltage input to a non-inverting input terminal andvoltage input to an inverting input terminal. The non-inverting inputterminal of the error amplifier 241 is the first terminal of the voltagecontrol circuit 244 and is electrically connected to the third terminalof the current control circuit 235. A second terminal of the voltagecontrol circuit 244 branches off from the first terminal of the voltagecontrol circuit 244 and is electrically connected to a first terminal ofthe current-voltage control switch circuit 252. Second reference voltageVref2 is input to the inverting input terminal of the error amplifier241. An output terminal of the error amplifier 241 is electricallyconnected to an inverting input terminal of the comparator 242.

The second reference voltage Vref2 input to the inverting input terminalof the error amplifier 241 and an inverting input terminal of ahysteresis comparator 251 to be described later is voltage for switchingbetween constant current charge and constant voltage charge. In otherwords, in the case where constant voltage charge is performed, constantcurrent charge is switched to constant voltage charge when the voltagereaches the second reference voltage Vref2.

The inverting input terminal of the comparator 242 is electricallyconnected to the output terminal of the error amplifier 241. A trianglewave is input to a non-inverting input terminal of the comparator 242from the triangle wave oscillation circuit 243. An output terminal ofthe comparator 242 is a third terminal of the voltage control circuit244 and is electrically connected to an input terminal B of themultiplexer 281.

The triangle wave oscillation circuit 243 generates a triangle wave. Inthe case where the voltage of the secondary battery 201 is high, thetriangle wave allows a reduction in time during which high voltage isapplied to the gate of the n-channel transistor 213. Accordingly,current used for charge can be small, which prevents an increase involtage of the secondary battery 201. The triangle wave generated by thetriangle wave oscillation circuit 243 is input to the non-invertinginput terminal of the comparator 242.

The current-voltage control switch circuit 252 includes the hysteresiscomparator 251. The current-voltage control switch circuit 252 has afunction of switching from current control to voltage control whenvoltage reaches a given value.

The hysteresis comparator 251 is a comparator in which hysteresis isadded to input and output. In other words, voltage at which output ischanged when a difference between voltage input to a non-inverting inputterminal and voltage input to an inverting input terminal is increasedis different from voltage at which output is changed when a differencebetween voltage input to the non-inverting input terminal and voltageinput to the inverting input terminal is decreased. With the hysteresiscomparator, frequent switching of output due to an influence of noisecan be prevented. A non-inverting input terminal of the hysteresiscomparator 251 is the first terminal of the current-voltage controlswitch circuit 252 and is electrically connected to the second terminalof the voltage control circuit 244. The second reference voltage Vref2is input to the inverting input terminal of the hysteresis comparator251. An output terminal of the hysteresis comparator 251 is a secondterminal of the current-voltage control switch circuit 252 and iselectrically connected to a first input terminal of the NAND circuit 284and an input terminal ϕ of the multiplexer 281.

The temperature control switch circuit 276 generates a signal(potential) to notify other circuits and elements of whether or notcharge is performed at the detected battery temperature or environmenttemperature in accordance with a signal (potential) from the NTCthermistor 221 which is a temperature detection element, and ofinformation on a method for controlling charge. The temperature controlswitch circuit 276 includes a resistor 261, a resistor 262, a resistor263, a resistor 264, a resistor 265, a hysteresis comparator 271, ahysteresis comparator 272, a hysteresis comparator 273, an inverter 274,and an inverter 275.

A power supply potential VL is input to one terminal of the resistor261, and the one terminal of the resistor 261 is electrically connectedto one terminal of the resistor 262. The other terminal of the resistor261 is the terminal THM of the control circuit 222 and a first terminalof the temperature control switch circuit 276, and is electricallyconnected to an inverting input terminal of the hysteresis comparator271, an inverting input terminal of the hysteresis comparator 272, andan inverting input terminal of the hysteresis comparator 273.

The one terminal of the resistor 262 is electrically connected to theone terminal of the resistor 261. The other terminal of the resistor 262is electrically connected to one terminal of the resistor 263 and anon-inverting input terminal of the hysteresis comparator 271.

The one terminal of the resistor 263 is electrically connected to theother terminal of the resistor 262 and the non-inverting input terminalof the hysteresis comparator 271. The other terminal of the resistor 263is electrically connected to one terminal of the resistor 264 and anon-inverting input terminal of the hysteresis comparator 272.

The one terminal of the resistor 264 is electrically connected to theother terminal of the resistor 263 and the non-inverting input terminalof the hysteresis comparator 272. The other terminal of the resistor 264is electrically connected to one terminal of the resistor 265 and anon-inverting input terminal of the hysteresis comparator 273.

The one terminal of the resistor 265 is electrically connected to theother terminal of the resistor 264 and the non-inverting input terminalof the hysteresis comparator 273. The other terminal of the resistor 265is grounded.

When resistance values of the resistor 262, the resistor 263, theresistor 264, and the resistor 265 are R1, R2, R3, and R4, respectively,potentials input to the non-inverting input terminals of the hysteresiscomparators 271 to 273 are each a divided potential of the power supplypotential VL, which is obtained by resistive division. The potentialinput to the non-inverting input terminal of the hysteresis comparator271 is (R2+R3+R4)(R1+R2+R3+R4)×VL. The potential input to thenon-inverting input terminal of the hysteresis comparator 272 is(R3+R4)/(R1+R2+R3+R4)×VL. The potential input to the non-inverting inputterminal of the hysteresis comparator 273 is R4/(R1+R2+R3+R4)×VL.

The voltage input to the inverting input terminals of the hysteresiscomparators 271 to 273 is a potential detected by the NTC thermistor221.

In each of the hysteresis comparators 271 to 273, the output is changeddepending on which potential, the potential input to the non-invertinginput terminal or the potential input to the inverting input terminal,is larger. Which output of the hysteresis comparators 271 to 273 is usedis determined in accordance with the temperature detected by the NTCthermistor 221.

The hysteresis comparator 271 detects the temperature of the NTCthermistor 221 higher than or equal to 40° C. The non-inverting inputterminal of the hysteresis comparator 271 is electrically connected tothe other terminal of the resistor 262 and the one terminal of theresistor 263. The inverting input terminal of the hysteresis comparator271 is the terminal THM of the control circuit 222 and the firstterminal of the temperature control switch circuit 276, and iselectrically connected to the other terminal of the resistor 261, theinverting input terminal of the hysteresis comparator 272, and theinverting input terminal of the hysteresis comparator 273. An outputterminal of the hysteresis comparator 271 is a second terminal of thetemperature control switch circuit 276 and is electrically connected toa second input terminal of the NAND circuit 284.

The hysteresis comparator 272 detects the temperature of the NTCthermistor 221 higher than or equal to 60° C. The non-inverting inputterminal of the hysteresis comparator 272 is electrically connected tothe other terminal of the resistor 263 and the one terminal of theresistor 264. The inverting input terminal of the hysteresis comparator272 is the terminal THM of the control circuit 222 and the firstterminal of the temperature control switch circuit 276, and iselectrically connected to the other terminal of the resistor 261, theinverting input terminal of the hysteresis comparator 271, and theinverting input terminal of the hysteresis comparator 273. An outputterminal of the hysteresis comparator 272 is electrically connected toan input terminal of the inverter 274.

The hysteresis comparator 273 detects the temperature of the NTCthermistor 221 higher than or equal to 90° C. The non-inverting inputterminal of the hysteresis comparator 273 is electrically connected tothe other terminal of the resistor 264 and the one terminal of theresistor 265. The inverting input terminal of the hysteresis comparator273 is the terminal THM of the control circuit 222 and the firstterminal of the temperature control switch circuit 276, and iselectrically connected to the other terminal of the resistor 261, theinverting input terminal of the hysteresis comparator 271, and theinverting input terminal of the hysteresis comparator 272. An outputterminal of the hysteresis comparator 273 is electrically connected toan input terminal of the inverter 275.

The input terminal of the inverter 274 is electrically connected to theoutput terminal of the hysteresis comparator 272. An output terminal ofthe inverter 274 is a third terminal of the temperature control switchcircuit 276 and is electrically connected to a first input terminal ofthe AND circuit 282 and an input terminal of the level shifter 285.

The input terminal of the inverter 275 is electrically connected to theoutput terminal of the hysteresis comparator 273. An output terminal ofthe inverter 275 is a fourth terminal of the temperature control switchcircuit 276 and is electrically connected to an input terminal of thelevel shifter 286.

The multiplexer 281 outputs a signal (potential) input to the inputterminal A or the input terminal B from an output terminal Y inaccordance with a signal (potential) input to the input terminal ϕ. Themultiplexer 281 in FIG. 11 outputs a signal input to the input terminalA from the output terminal Y when a signal input to the input terminal 4has a low-level potential. The multiplexer 281 outputs a signal input tothe input terminal B from the output terminal Y when a signal input tothe input terminal ϕ has a high-level potential.

The input terminal A of the multiplexer 281 is electrically connected tothe fourth terminal of the current control circuit 235. The inputterminal B of the multiplexer 281 is electrically connected to the thirdterminal of the voltage control circuit 244. The input terminal ϕ of themultiplexer 281 is electrically connected to the second terminal of thecurrent-voltage control switch circuit 252 and the first input terminalof the NAND circuit 284. The output terminal Y of the multiplexer 281 iselectrically connected to a third input terminal of the AND circuit 282.

The first input terminal of the AND circuit 282 is electricallyconnected to the third terminal of the temperature control switchcircuit 276 and the input terminal of the level shifter 285. A secondinput terminal of the AND circuit 282 is electrically connected to anoutput terminal of the NAND circuit 284. The third input terminal of theAND circuit 282 is electrically connected to the output terminal Y ofthe multiplexer 281.

The level shifter 283 has a function of changing the voltage range of asignal. An input terminal of the level shifter 283 is electricallyconnected to an output terminal of the AND circuit 282. An outputterminal of the level shifter 283 is the terminal GS of the controlcircuit 222.

The first input terminal of the NAND circuit 284 is electricallyconnected to the second terminal of the current-voltage control switchcircuit 252 and the input terminal ϕ of the multiplexer 281. The secondinput terminal of the NAND circuit 284 is electrically connected to thesecond terminal of the temperature control switch circuit 276. Theoutput terminal of the NAND circuit 284 is electrically connected to thesecond input terminal of the AND circuit 282.

The input terminal of the level shifter 285 is electrically connected tothe third terminal of the temperature control switch circuit 276 and thefirst input terminal of the AND circuit 282. An output terminal of thelevel shifter 285 is the terminal CHA of the control circuit 222.

The input terminal of the level shifter 286 is electrically connected tothe fourth terminal of the temperature control switch circuit 276. Anoutput terminal of the level shifter 286 is the terminal DIS of thecontrol circuit 222.

In the battery charger 200 of this embodiment, other than the lithiumion secondary battery 130, another lithium ion secondary battery oranother secondary battery such as a lead-acid battery or a nickel-metalhydride battery can be used as the secondary battery 201. Further, inthe battery charger of this embodiment, a capacitor (e.g., a lithium ioncapacitor or an electrical double-layer capacitor) can be used insteadof the secondary battery 201.

<<Operation of Battery Charger>>

In the battery charger 200 illustrated in FIG. 10 and FIG. 11, thesecondary battery 201 is charged in the following manner.

Charge current with a given current value flows to the secondary battery201 from the direct-current power source 216. At this time, the voltagefrom the direct-current power source 216 is converted by the powerconversion circuit 215 so as to have a voltage value at Which thesecondary battery 201 can be charged.

In a normal time, the n-channel transistor 204 of the charge controlswitch 205 and the n-channel transistor 207 of the discharge controlswitch 208 are in an on state. Therefore, the charge current flows intothe secondary battery 201 through the discharge control switch 208, thecharge control switch 205, and the resistor 202 and constant currentcharge proceeds. At this time, the current value of the charge currentflowing into the secondary battery 201 is determined by the voltagevalue and the resistance of the resistor 202.

The voltage applied to the NTC thermistor 221 placed in the vicinity ofthe secondary battery 201 and detecting the environment temperature ofthe secondary battery 201 is input to the terminal THM of the controlcircuit 222 and then to the inverting input terminals of the hysteresiscomparators 271 to 273. The hysteresis comparators 271 to 273 eachcompare the voltage input to the non-inverting input terminal with thevoltage input to the inverting input terminal, and outputs of thehysteresis comparators 271 to 273 which detect the correspondingtemperature are inverted.

Table 1 shows whether charge is performed or not, a method forcontrolling charge, and whether discharge is performed or not in eachtemperature range in the battery charger 200 illustrated in FIG. 10 andFIG. 11.

TABLE 1 40° C. or 60° C. or higher and higher and Lower lower than lowerthan 90° C. or than 40° C. 60° C. 90° C. higher Charge Current ∘ ∘ x xcontrol Voltage ∘ x x x control Discharge ∘ ∘ ∘ x<<<Lower than 40° C.>>>

In the case where outputs of all the hysteresis comparators 271 to 273are not inverted, that is, the temperature detected by the NTCthermistor 221 is lower than 40° C., the outputs of the hysteresiscomparators 271 to 273 are low-level potentials.

Since the output of the hysteresis comparator 271 is a low-levelpotential, a potential input to the second input terminal of the NANDcircuit 284 is also a low-level potential.

As described above, the voltage of the secondary battery 201 is thevoltage applied to the terminal CSIN. In the hysteresis comparator 251of the current-voltage control switch circuit 252, the voltage of thesecondary battery 201 is input to the non-inverting input terminalthrough the terminal CSIN and the second reference voltage Vref2 isinput to the inverting input terminal. As described above, the secondreference voltage Vref2 is voltage for switching between constantcurrent charge and constant voltage charge in the case where constantvoltage charge is performed. When the voltage of the secondary battery201, which is input to the non-inverting input terminal, exceeds thesecond reference voltage Vref2, the output of the hysteresis comparator251 is inverted from a low-level potential to a high-level potential.

When the voltage of the secondary battery 201 does not exceed the secondreference voltage Vref2 and the output of the hysteresis comparator 251is a low-level potential a potential input to the input terminal ϕ ofthe multiplexer 281 is a low-level potential.

As described above, the multiplexer 281 outputs a signal (potential)input to the input terminal A or the input terminal B from the outputterminal Y in accordance with a signal (potential) input to the inputterminal ϕ. The multiplexer 281 in FIG. 11 outputs a signal input to theinput terminal A from the output terminal Y when a signal input to theinput terminal ϕ is a low-level potential. The multiplexer 281 outputs asignal input to the input terminal B from the output terminal Y when asignal input to the input terminal ϕ is a high-level potential.

Therefore, since a signal which is output from the hysteresis comparator251 and input to the input terminal ϕ of the multiplexer 281 is alow-level potential, a signal output from the output terminal Y of themultiplexer 281 is a signal input to the terminal A, that is, a signalfrom the current control circuit 235. The signal from the currentcontrol circuit 235, which is output from the output terminal Y of themultiplexer 281, is input to the third input terminal of the AND circuit282.

Here, output from the output terminal of the NAND circuit 284 is inputto the second input terminal of the AND circuit 282. The output of thehysteresis comparator 251 is input to the first input terminal of theNAND circuit 284. As described above, since the output of the hysteresiscomparator 251 is a low-level potential, a potential input to the firstinput terminal of the NAND circuit 284 is also a low-level potential.

Further, as described above, since the output of the hysteresiscomparator 271 is a low-level potential, a potential input to the secondinput terminal of the NAND circuit 284 is also a low-level potential.

Therefore, a potential output from the output terminal of the NANDcircuit 284 is a high-level potential. Thus, the high-level potentialoutput from the output terminal of the NAND circuit 284 is input to thesecond input terminal of the AND circuit 282.

A potential (high-level potential) obtained by inverting the output(low-level potential) of the hysteresis comparator 272 by the inverter274 is input to the first input terminal of the AND circuit 282.

Accordingly, a high-level potential is input to the first input terminalof the AND circuit 282. A high-level potential is input to the secondinput terminal of the AND circuit 282. The output of the current controlcircuit 235 is input to the third input terminal of the AND circuit 282through the input terminal A of the multiplexer 281. In other words, theoutput of the AND circuit 282 corresponds to the output of the currentcontrol circuit 235.

The voltage range of the signal output from the AND circuit 282 ischanged by the level shifter 283, and the output of the AND circuit 282is applied to the gate of the n-channel transistor 213 of the powerconversion circuit 215.

Depending on the output of the current control circuit 235, then-channel transistor 213 is in an on state or an off state, so that thecharge of the secondary battery 201 by the direct-current power source216 is controlled.

As described above, in the case where the temperature detected by theNTC thermistor 221 is lower than 40° C. and the voltage of the secondarybattery 201 does not exceed the second reference voltage Vref2, thecurrent control circuit 235 operates and constant current charge isperformed.

When the voltage of the secondary battery 201, which is input to thenon-inverting input terminal of the hysteresis comparator 251, exceedsthe second reference voltage Vref2, the output of the hysteresiscomparator 251 is inverted from a low-level potential to a high-levelpotential.

The output of the hysteresis comparator 251 which becomes a high-levelpotential is input to the input terminal ϕ, of the multiplexer 281. Inthe case where a high-level potential is input to the input terminal ϕof the multiplexer 281, a signal input to the input terminal B, that is,a signal from the voltage control circuit 244, is output from the outputterminal Y of the multiplexer 281. The signal from the voltage controlcircuit 244, which is output from the output terminal Y of themultiplexer 281, is input to the third input terminal of the AND circuit282.

As described above, in the case where the temperature detected by theNTC thermistor 221 is lower than 40° C., the output of the hysteresiscomparator 271 is a low-level potential. Accordingly, a potential inputto the second input terminal of the NAND circuit 284 is also a low-levelpotential.

A potential input to the first input terminal of the NAND circuit 284 isthe output of the hysteresis comparator 251 and a high-level potential.A potential input to the second input terminal of the NAND circuit 284is a low-level potential as described above. Accordingly, a high-levelpotential is output from the output terminal of the NAND circuit 284.

A signal from the voltage control circuit 244 is input to the thirdinput terminal of the AND circuit 282.

A high-level potential which is output from the output terminal of theNAND circuit 284 is input to the second input terminal of the ANDcircuit 282.

A potential (high-level potential) obtained by inverting the output(low-level potential) of the hysteresis comparator 272 by the inverter274 is input to the first input terminal of the AND circuit 282.

Accordingly, a high-level potential is input to the first input terminalof the AND circuit 282. A high-level potential is input to the secondinput terminal of the AND circuit 282. The output of the voltage controlcircuit 244 is input to the third input terminal of the AND circuit 282through the input terminal B of the multiplexer 281. In other words, theoutput of the AND circuit 282 corresponds to the output of the voltagecontrol circuit 244.

The output of the AND circuit 282 whose voltage range is changed by thelevel shifter 283 is applied to the gate of the n-channel transistor 213of the power conversion circuit 215.

Depending on the output of the voltage control circuit 244, then-channel transistor 213 is in an on state or an off state, so thatcharge of the secondary battery 201 by the direct-current power source216 is controlled.

Thus, in the case where the temperature detected by the NTC thermistor221 is lower than 40° C. and the voltage of the secondary battery 201 ishigher than or equal to the second reference voltage Vref2, the voltagecontrol circuit 244 operates and constant voltage charge is performed.

<<<Higher than or Equal to 40° C. and Lower than 60° C.>>>

In the case where only the output of the hysteresis comparator 271 isinverted from a low-level potential to a high-level potential and theoutputs of the hysteresis comparators 272 and 273 are not inverted(low-level potential), that is, the temperature detected by the NTCthermistor 221 is higher than or equal to 40° C. and lower than 60° C.,the inverted output (high-level potential) of the hysteresis comparator271 is input to the second input terminal of the NAND circuit 284.

As described above, the voltage of the secondary battery 201 is thevoltage applied to the terminal CSIN. In the hysteresis comparator 251of the current-voltage control switch circuit 252, the voltage of thesecondary battery 201 is input to the non-inverting input terminalthrough the terminal CSIN and the second reference voltage Vref2 isinput to the inverting input terminal. As described above, the secondreference voltage Vref2 is voltage for switching between constantcurrent charge and constant voltage charge in the case where constantvoltage charge is performed. When the voltage of the secondary battery201, which is input to the non-inverting input terminal, exceeds thesecond reference voltage Vref2, the output of the hysteresis comparator251 is inverted from a low-level potential to a high-level potential.

When the voltage of the secondary battery 201 does not exceed the secondreference voltage Vref2 and the output of the hysteresis comparator 251is a low-level potential, a potential input to the input terminal ϕ ofthe multiplexer 281 is a low-level potential.

As described above, the multiplexer 281 in FIG. 11 outputs a potentialinput to the input terminal A, from the output terminal Y when apotential input to the input terminal ϕ is a low-level potential. Themultiplexer 281 outputs a potential input to the input terminal B fromthe output terminal Y when a potential input to the input terminal ϕ isa high-level potential.

Therefore, in the case where the voltage of the secondary battery 201does not exceed the second reference voltage Vref2 and the output of thehysteresis comparator 251 is a low-level potential, a signal output fromthe output terminal Y of the multiplexer 281 is a signal input to theterminal A, that is, a signal from the current control circuit 235. Thesignal from the current control circuit 235, which is output from theoutput terminal Y of the multiplexer 281, is input to the third inputterminal of the AND circuit 282.

Here, output from the output terminal of the NAND circuit 284 is inputto the second input terminal of the AND circuit 282. The output of thehysteresis comparator 251 is input to the first input terminal of theNAND circuit 284. As described above, since the output of the hysteresiscomparator 251 is a low-level potential, a potential input to the firstinput terminal of the NAND circuit 284 is also a low-level potential.

Further, as described above, since the output of the hysteresiscomparator 271 is a high-level potential, a potential input to thesecond input terminal of the NAND circuit 284 is also a high-levelpotential.

Therefore, a potential output from the output terminal of the NANDcircuit 284 is a high-level potential. Thus, the high-level potentialoutput from the output terminal of the NAND circuit 284 is input to thesecond input terminal of the AND circuit 282.

A potential (high-level potential) obtained by inverting the output(low-level potential) of the hysteresis comparator 272 by the inverter274 is input to the first input terminal of the AND circuit 282.

Accordingly, a high-level potential is input to the first input terminalof the AND circuit 282. A high-level potential is input to the secondinput terminal of the AND circuit 282. The output of the current controlcircuit 235 is input to the third input terminal of the AND circuit 282through the input terminal A of the multiplexer 281. In other words, theoutput of the AND circuit 282 corresponds to the output of the currentcontrol circuit 235.

The voltage range of the signal output from the AND circuit 282 ischanged by the level shifter 283, and the output of the AND circuit 282is applied to the gate of the n-channel transistor 213 of the powerconversion circuit 215.

Depending on the output of the current control circuit 235, then-channel transistor 213 is in an on state or an off state, so that thecharge of the secondary battery 201 by the direct-current power source216 is controlled.

As described above, in the case where the temperature detected by theNTC thermistor 221 is higher than or equal to 40° C. and lower than 60°C. and the voltage of the secondary battery 201 does not exceed thesecond reference voltage Vref2, the current control circuit 235 operatesand constant current charge is performed.

When the voltage of the secondary battery 201, which is input to thenon-inverting input terminal of the hysteresis comparator 251, exceedsthe second reference voltage Vref2, the output of the hysteresiscomparator 251 is inverted from a low-level potential to a high-levelpotential.

The output of the hysteresis comparator 251, which is inverted to ahigh-level potential, is input to the first input terminal of the NANDcircuit 284.

Since the output of the hysteresis comparator 271 is a high-levelpotential, a potential input to the second input terminal of the NANDcircuit 284 is also a high-level potential. Accordingly, a low-levelpotential is output from the NAND circuit 284.

A low-level potential is output from the NAND circuit 284, so that alow-level potential is input to the second input terminal of the ANDcircuit 282 and a low-level potential is output from the output terminalof the AND circuit 282.

The low-level potential which is the output of the AND circuit 282 isapplied to the gate of the n-channel transistor 213 of the powerconversion circuit 215 through the level shifter 283.

A low-level potential is applied to the gate of the n-channel transistor213 of the power conversion circuit 215, so that the n-channeltransistor 213 is turned off. Accordingly, charge of the secondarybattery 201 by the direct-current power source 216 is stopped and chargeis terminated.

<<<Higher than or Equal to 60° C. and Lower than 90° C.>>>

In the case where the outputs of the hysteresis comparators 271 and 272are inverted from a low-level potential to a high-level potential andthe output of the hysteresis comparator 273 is not inverted (low-levelpotential), that is, the temperature detected by the NTC thermistor 221is higher than or equal to 60° C. and lower than 90° C., a high-levelpotential which is the output of the hysteresis comparator 272 isinverted to a low-level potential by the inverter 274.

The low-level potential output from the inverter 274 is input to thefirst input terminal of the AND circuit 282. Accordingly, a low-levelpotential is output from the output terminal of the AND circuit 282.

The low-level potential which is the output of the AND circuit 282 isapplied to the gate of the n-channel transistor 213 of the powerconversion circuit 215 through the level shifter 283.

A low-level potential is applied to the gate of the n-channel transistor213 of the power conversion circuit 215, so that the n-channeltransistor 213 is turned off. Accordingly, charge of the secondarybattery 201 by the direct-current power source 216 is suppressed.

Further, the low-level potential output from the inverter 274 is inputto the charge control switch 205 through the terminal CHA and then-channel transistor 204 is turned off due to voltage applied to thegate of the n-channel transistor 204. Thus, charge of the secondarybattery 201 is suppressed.

<<<Higher than or Equal to 90° C.>>>

In the case where the outputs of the hysteresis comparators 271 to 273are inverted from a low-level potential to a high-level potential, thatis, the temperature detected by the NTC thermistor 221 is higher than orequal to 90° C., a high-level potential which is the output of thehysteresis comparator 272 is inverted by the inverter 274 and then alow-level potential is input to the first input terminal of the ANDcircuit. Thus, charge of the secondary battery 201 by the direct-currentpower source 216 is suppressed.

In the case where the output of the hysteresis comparator 273 isinverted from a low-level potential to a high-level potential, thehigh-level potential which is the output of the hysteresis comparator273 is inverted by the inverter 275 to a low-level potential. Thelow-level potential is input to the discharge control switch 208 throughthe terminal DIS to be applied to the gate of the n-channel transistor207. Accordingly, the n-channel transistor 207 is turned off and notonly charge of the secondary battery 201 but also discharge of thesecondary battery 201 is suppressed.

As described above, in the battery charger 200 illustrated in FIG. 10and FIG. 11, whether charge is possible or not is determined inaccordance with the battery temperature or the environment temperatureof the secondary battery 201 detected by the NTC thermistor 221 which isa temperature detection element. Further, in the case where charge isperformed, it is determined to perform either constant current charge orconstant voltage charge. Further, in the battery charger 200 illustratedin FIG. 10 and FIG. 11, whether discharge is possible or not is alsocontrolled in accordance with the battery temperature or the environmenttemperature of the secondary battery 201.

With the use of the above battery charger, an electrode of a lithium ionsecondary battery can be prevented from deteriorating.

Further, with the use of the above battery charger, deterioration ofbattery characteristics of a lithium ion secondary battery can beprevented.

<Method for Manufacturing Lithium Ion Secondary Battery>

First, a method for forming the positive electrode 102 is describedbelow.

Slurry containing particles of a positive electrode active material(lithium iron phosphate), a binding agent, and a conductive additive isformed. Then, the slurry is applied over a surface of the positiveelectrode current collector 100. The positive electrode currentcollector 100 over which the slurry is applied is heated, so that thepositive electrode active material is baked. Thus, the positiveelectrode active material layer 101 is formed over the positiveelectrode current collector 100.

The case where a positive electrode active material layer is coveredwith graphene is now described.

First, slurry containing particles of a positive electrode activematerial (lithium iron phosphate) and graphene oxide is formed. Abinding agent and a conductive additive may be added to the slurry ifneeded. Then, the slurry is applied over the surface of the positiveelectrode current collector 100. After that, heating as reductiontreatment is performed in a reduced atmosphere, so that the positiveelectrode active material is baked and oxygen contained in the grapheneoxide is released to form openings in the graphene. Note that oxygen inthe graphene oxide is not entirely released and partly remains in thegraphene.

Through the above process, the positive electrode active material layer101 can be formed over the positive electrode current collector 100.Consequently, the positive electrode active material layer 101 hashigher conductivity. Graphene oxide contains oxygen and thus isnegatively charged in a polar solvent. As a result of being negativelycharged, graphene oxide is dispersed. Accordingly, the positiveelectrode active materials contained in the slurry are not easilyaggregated, so that the size of the particle of the positive electrodeactive material can be prevented from increasing due to baking. Thus,the transfer of electrons in the positive electrode active materials isfacilitated, resulting in an increase in conductivity of the positiveelectrode active material layer 101.

A method for forming the negative electrode 107 is described below.

Slurry containing particles of a negative electrode active material(graphite), a binding agent, and a conductive additive is formed. Then,the slurry is applied over a surface of the negative electrode currentcollector 105. The negative electrode current collector 105 over whichthe slurry is applied is heated, so that the negative electrode activematerial is baked. Thus, the negative electrode active material layer106 is formed over the negative electrode current collector 105.

The case where the negative electrode active material layer 106 iscovered with graphene is now described.

First, slurry containing particles of a negative electrode activematerial and graphene oxide is formed. Specifically, the particles ofthe negative electrode active material and a dispersion liquidcontaining graphene oxide are mixed to form the slurry. A binding agentand a conductive additive may be added to the slurry if needed.

Next, the slurry is applied over the negative electrode currentcollector 105. Then, vacuum drying is performed for a certain time forremoving the solvent from the slurry applied over the negative electrodecurrent collector 105.

After that, the graphene oxide is electrochemically reduced withelectric energy to form graphene. Through the above process, thenegative electrode active material layer 106 can be formed over thenegative electrode current collector 105.

Note that regardless of whether or not the negative electrode activematerial layer 106 is covered with graphene, the negative electrodeactive material layer 106 may be predoped with lithium. Predoping withlithium may be performed in such a manner that a lithium layer is formedon a surface of the negative electrode active material layer 106 by asputtering method. Alternatively, lithium foil is provided on thesurface of the negative electrode active material layer 106, whereby thenegative electrode active material layer 106 can be predoped withlithium.

<Another Structure of Lithium Ion Secondary Battery>

FIGS. 5A and 5B illustrate a lithium ion secondary battery having astructure different from that in FIG. 1.

FIG. 5A is a top view of a lithium ion secondary battery 151. Thelithium ion secondary battery 151 illustrated in FIG. 5A includes apower storage cell 155 in an exterior member 153. The lithium ionsecondary battery 151 further includes terminal portions 157 and 159which are connected to the power storage cell 155. For the exteriormember 153, a laminate film, a polymer film, a metal film, a metal case,a plastic case, or the like can be used.

FIG. 5B illustrates a cross section of the lithium ion secondary battery151 along line X-Y illustrated in FIG. 5A. As illustrated in FIG. 5B,the power storage cell 155 includes a negative electrode 163, a positiveelectrode 165, a separator 167 between the negative electrode 163 andthe positive electrode 165, and an electrolyte 169 with which theexterior member 153 is filled. In the lithium ion secondary battery 151illustrated in FIGS. 5A and 5B, the positive electrode 165, the negativeelectrode 163, and the separator 167 are stacked.

Although a sealed thin lithium ion secondary battery is described as thelithium ion secondary battery 151 in this embodiment, the external shapeof the lithium ion secondary battery 151 is not limited thereto. Theexternal shape of the lithium ion secondary battery 151 may be varied;for example, a button-type lithium ion secondary battery, a cylindricallithium ion secondary battery, a square-type lithium ion secondarybattery, or the like may be employed. Further, although the structurewhere the positive electrode 165, the negative electrode 163, and theseparator 167 are stacked is illustrated in FIGS. 5A and 5B, a structurewhere a positive electrode, a negative electrode, and a separator arerolled may be employed.

A positive electrode current collector 175 is connected to the terminalportion 157. A negative electrode current collector 171 is connected tothe terminal portion 159. Further, the terminal portion 157 and theterminal portion 159 each partly extend outside the exterior member 153.

The positive electrode 165 includes the positive electrode currentcollector 175 and a positive electrode active material layer 177. Thepositive electrode active material layer 177 is formed on one or bothsurfaces of the positive electrode current collector 175. The positiveelectrode active material layer 177 may include a binding agent and aconductive additive.

The positive electrode current collector 175 may be formed into a shapesimilar to and using a material similar to those of the positiveelectrode current collector 100 in FIG. 1. Further, the positiveelectrode active material layer 177 may be formed using a material and aformation method similar to those of the positive electrode activematerial layer 101 in FIG. 1

Note that a simple substance of any of the above materials applicable tothe positive electrode active material layer 177 may be used as thepositive electrode without using the positive electrode currentcollector 175.

The negative electrode 163 includes the negative electrode collector 171and a negative electrode active material layer 173. The negativeelectrode active material layer 173 is formed on one or both surfaces ofthe negative electrode current collector 171. In addition, the negativeelectrode active material layer 173 may include a binding agent and aconductive additive.

The negative electrode current collector 171 may be formed into a shapesimilar to and using a material similar to those of the negativeelectrode current collector 105 in FIG. 1. Further, the negativeelectrode active material layer 173 may be formed using a material and aformation method similar to those of the negative electrode activematerial layer 106 in FIG. 1

Note that a simple substance of any of the above materials applicable tothe negative electrode active material layer 173 may be used as thenegative electrode without using the negative electrode currentcollector 171.

The electrolyte 169 includes a solute and a solvent. For the solute andthe solvent included in the electrolyte 169, materials similar to thoseused for the electrolyte 111 in FIG. 1 can be used.

For the separator 167, a material similar to that of the separator 110in FIG. 1 can be used.

<Electric Device>

A lithium ion secondary battery of an embodiment of the presentinvention can be used for power supplies of a variety of electricdevices.

Specific examples of electric devices each utilizing the lithium ionsecondary battery of an embodiment of the present invention are asfollows: display devices such as televisions and monitors, lightingdevices, desktop or laptop personal computers, word processors, imagereproduction devices which reproduce still images or moving imagesstored in recording media such as digital versatile discs (DVDs),portable compact disc (CD) players, radio receivers, tape recorders,headphone stereos, stereos, clocks such as table clocks and wall clocks,cordless phone handsets, transceivers, portable wireless devices,cellular phones, car phones, portable game machines, calculators,portable information terminals, electronic notebooks, e-book readers,electronic translators, audio input devices, cameras such as stillcameras and video cameras, electric shavers, high-frequency heatingappliances such as microwave ovens, electric rice cookers, electricwashing machines, electric vacuum cleaners, water heaters, electricfans, hair dryers, air-conditioning systems such as humidifiers,dehumidifiers, and air conditioners, dishwashing machines, dish dryingmachines, clothes dryers, futon dryers, electric refrigerators, electricfreezers, electric refrigerator-freezers, freezers for preserving DNA,flashlights, electric power tools such as chain saws, smoke detectors,and medical equipment such as dialyzers. The examples also includeindustrial equipment such as guide lights, traffic lights, beltconveyors, elevators, escalators, industrial robots, power storagesystems, and power storage devices for leveling the amount of powersupply and smart grid. In addition, moving objects driven by electricmotors using power from lithium ion secondary batteries are alsoincluded in the category of electric devices. As examples of the movingobjects, there are electric vehicles (EV), hybrid electric vehicles(HEV) which include both an internal-combustion engine and a motor,plug-in hybrid electric vehicles (PHEV), tracked vehicles in whichcaterpillar tracks are substituted for wheels of these vehicles,motorized bicycles including motor-assisted bicycles, motorcycles,electric wheelchairs, golf carts, boats, ships, submarines, helicopters,aircrafts, rockets, artificial satellites, space probes, planetaryprobes, and spacecrafts.

In the above electric devices, the lithium ion secondary battery of anembodiment of the present invention can be used as a main power supplyfor supplying enough power for almost the whole power consumption.Alternatively, in the above electric devices, the lithium ion secondarybattery of an embodiment of the present invention can be used as anuninterruptible power supply which can supply power to the electricdevices when the supply of power from the main power supply or acommercial power supply is stopped. Still alternatively, in the aboveelectric devices, the lithium ion secondary battery of an embodiment ofthe present invention can be used as an auxiliary power supply forsupplying power to the electric devices at the same time as the powersupply from the main power supply or a commercial power supply.

FIG. 6 illustrates specific structures of the above electric devices. InFIG. 6, a display device 8000 is an example of an electric deviceincluding a lithium ion secondary battery 8004 of an embodiment of thepresent invention. Specifically, the display device 8000 corresponds toa display device for TV broadcast reception and includes a housing 8001,a display portion 8002, speaker portions 8003, the lithium ion secondarybattery 8004, and the like. The lithium ion secondary battery 8004 of anembodiment of the present invention is provided inside the housing 8001.The display device 8000 can receive power from a commercial powersupply, or can use power stored in the lithium ion secondary battery8004. Thus, the display device 8000 can be operated with the use of thelithium ion secondary battery 8004 of an embodiment of the presentinvention as an uninterruptible power supply even when power cannot besupplied from a commercial power supply due to power failure or thelike.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoresis displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), or a field emission display (FED) can be used for the displayportion 8002.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like besides TV broadcast reception.

In FIG. 6, an installation lighting device 8100 is an example of anelectric device including a lithium ion secondary battery 8103 of anembodiment of the present invention. Specifically, the lighting device8100 includes a housing 8101, a light source 8102, the lithium ionsecondary battery 8103, and the like. Although FIG. 6 illustrates thecase where the lithium ion secondary battery 8103 is provided inside aceiling 8104 on which the housing 8101 and the light source 8102 areinstalled, the lithium ion secondary battery 8103 may be provided insidethe housing 8101. The lighting device 8100 can receive power from acommercial power supply, or can use power stored in the lithium ionsecondary battery 8103. Thus, the lighting device 8100 can be operatedwith the use of the lithium ion secondary battery 8103 of an embodimentof the present invention as an uninterruptible power supply even whenpower cannot be supplied from a commercial power supply due to powerfailure or the like.

Although the installation lighting device 8100 provided in the ceiling8104 is illustrated in FIG. 6 as an example, the lithium ion secondarybattery of an embodiment of the present invention can be used for aninstallation lighting device provided in, for example, a wall 8105, afloor 8106, a window 8107, or the like other than the ceiling 8104.Further, the lithium ion secondary battery can be used in a tabletoplighting device or the like.

As the light source 8102, an artificial light source which emits lightartificially by using power can be used. Specifically, an incandescentlamp, a discharge lamp such as a fluorescent lamp, and light-emittingelements such as an LED or an organic EL element are given as examplesof the artificial light source.

In FIG. 6, an air conditioner including an indoor unit 8200 and anoutdoor unit 8204 is an example of an electric device including alithium ion secondary battery 8203 of an embodiment of the presentinvention. Specifically, the indoor unit 8200 includes a housing 8201,an air outlet 8202, the lithium ion secondary battery 8203, and thelike. Although FIG. 6 illustrates the case where the lithium ionsecondary battery 8203 is provided in the indoor unit 8200, the lithiumion secondary battery 8203 may be provided in the outdoor unit 8204.Alternatively, the lithium ion secondary battery 8203 may be provided inboth the indoor unit 8200 and the outdoor unit 8204. The air conditionercan receive power from a commercial power supply, or can use powerstored in the lithium ion secondary battery 8203. Particularly in thecase where the lithium ion secondary battery 8203 is provided in boththe indoor unit 8200 and the outdoor unit 8204, the air conditioner canbe operated with the use of the lithium ion secondary battery 8203 of anembodiment of the present invention as an uninterruptible power supplyeven when power cannot be supplied from a commercial power supply due topower failure or the like.

Although the split-type air conditioner including the indoor unit andthe outdoor unit is illustrated in FIG. 6 as an example, the lithium ionsecondary battery of an embodiment of the present invention can be usedin an air conditioner in which the functions of an indoor unit and anoutdoor unit are integrated in one housing.

In FIG. 6, an electric refrigerator-freezer 8300 is an example of anelectric device including a lithium ion secondary battery 8304 of anembodiment of the present invention. Specifically, the electricrefrigerator-freezer 8300 includes a housing 8301, a door for arefrigerator 8302, a door for a freezer 8303, the lithium ion secondarybattery 8304, and the like. The lithium ion secondary battery 8304 isprovided inside the housing 8301 in FIG. 6. The electricrefrigerator-freezer 8300 can receive power from a commercial powersupply, or can use power stored in the lithium ion secondary battery8304. Thus, the electric refrigerator-freezer 8300 can be operated withthe use of the lithium ion secondary battery 8304 of an embodiment ofthe present invention as an uninterruptible power supply even when powercannot be supplied from a commercial power supply due to power failureor the like.

Note that among the electric devices described above, a high-frequencyheating device such as a microwave oven and an electric device such asan electric rice cooker require high power in a short time. The trippingof a breaker of a commercial power supply in use of an electric devicecan be prevented by using the lithium ion secondary battery of anembodiment of the present invention as an auxiliary power supply forsupplying power which cannot be supplied enough by the commercial powersupply.

In addition, in a time period when electric devices are not used,particularly when the proportion of the amount of power which isactually used to the total amount of power which can be supplied from acommercial power supply source (such a proportion referred to as a usagerate of power) is low, power can be stored in the lithium ion secondarybattery, whereby the usage rate of power can be reduced in a time periodwhen the electric devices are used. For example, in the case of theelectric refrigerator-freezer 8300, power can be stored in the lithiumion secondary battery 8304 in night time when the temperature is low andthe door for a refrigerator 8302 and the door for a freezer 8303 are notoften opened or closed. On the other hand, in daytime when thetemperature is high and the door for a refrigerator 8302 and the doorfor a freezer 8303 are frequently opened and closed, the lithium ionsecondary battery 8304 is used as an auxiliary power supply; thus, theusage rate of power in daytime can be reduced.

Next, a portable information terminal which is another example of theelectric devices will be described with reference to FIGS. 7A to 7C.

FIGS. 7A and 7B illustrate a tablet terminal that can be folded. In FIG.7A, the tablet terminal is opened, and includes a housing 9630, adisplay portion 9631 a, a display portion 9631 b, a display-modeswitching button 9034, a power button 9035, a power-saving-modeswitching button 9036, a clip 9033, and an operation button 9038.

A touch panel area 9632 a can be provided in a part of the displayportion 9631 a, in which area, data can be input by touching displayedoperation keys 9638. Half of the display portion 9631 a has only adisplay function and the other half has a touch panel function; however,an embodiment of the present invention is not limited to this structure,and the whole display portion 9631 a may have a touch panel function.For example, a keyboard can be displayed on the whole display portion9631 a to be used as a touch panel, and the display portion 9631 b canbe used as a display screen.

A touch panel area 9632 b can be provided in a part of the displayportion 9631 b like in the display portion 9631 a. When a keyboarddisplay switching button 9639 displayed on the touch panel is touchedwith a finger, a stylus, or the like, a keyboard can be displayed on thedisplay portion 9631 b.

The touch panel area 9632 a and the touch panel area 9632 b can becontrolled by touch input at the same time.

The display-mode switching button 9034 allows switching between alandscape mode and a portrait mode, color display and black-and-whitedisplay, and the like. The power-saving-mode switching button 9036allows optimizing the display luminance in accordance with the amount ofexternal light in use which is detected by an optical sensorincorporated in the tablet terminal. In addition to the optical sensor,other detecting devices such as sensors for detecting inclination, likea gyroscope or an acceleration sensor, may be incorporated in the tabletterminal.

Although the display portion 9631 a and the display portion 9631 b havethe same display area in FIG. 7A, an embodiment of the present inventionis not limited to this example. The display portion 9631 a and thedisplay portion 9631 b may have different areas or different displayquality. For example, higher definition images may be displayed on oneof the display portions 9631 a and 9631 b.

FIG. 7B illustrates the tablet terminal folded, which includes thehousing 9630, a solar battery 9633, a charge and discharge controlcircuit 9634, a battery 9635, and a DCDC converter 9636. Note that FIG.7B shows an example in which the charge and discharge control circuit9634 includes the battery 9635 and the DCDC converter 9636, and thebattery 9635 includes the lithium ion secondary battery described in anyof the above embodiments.

Since the tablet terminal can be folded, the housing 9630 can be closedwhen not in use. Thus, the display portions 9631 a and 9631 b can beprotected, which makes it possible to provide a tablet terminal withhigh durability and improved reliability for long-term use.

The tablet terminal illustrated in FIGS. 7A and 7B can have otherfunctions such as a function of displaying various kinds of data (e.g.,a still image, a moving image, and a text image), a function ofdisplaying a calendar, a date, the time, or the like on the displayportion, a touch-input function of operating or editing the datadisplayed on the display portion by touch input, and a function ofcontrolling processing by various kinds of software (programs).

The solar battery 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processor, and the like. Note that the solar battery9633 can be provided on one or both surfaces of the housing 9630 so thatthe battery 9635 can be charged efficiently. The use of the lithium ionsecondary battery of an embodiment of the present invention as thebattery 9635 is advantageous in downsizing or the like.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 7B are described with reference to a blockdiagram of FIG. 7C. FIG. 7C illustrates the solar battery 9633, thebattery 9635, the DCDC converter 9636, a converter 9637, switches SW1 toSW3, and the display portion 9631. The battery 9635, the DCDC converter9636, the converter 9637, and the switches SW1 to SW3 correspond to thecharge and discharge control circuit 9634 in FIG. 7B.

First, description is made on an example of the operation in the casewhere power is generated by the solar battery 9633 using external light.The voltage of power generated by the solar battery 9633 is raised orlowered by the DCDC converter 9636 so that a voltage for charging thebattery 9635 is obtained. When the display portion 9631 is operated withthe power from the solar battery 9633, the switch SW1 is turned on andthe voltage of the power is raised or lowered by the converter 9637 to avoltage needed for operating the display portion 9631. When display isnot performed on the display portion 9631, the switch SW1 is turned offand the switch SW2 is turned on so that the battery 9635 can be charged.

Although the solar battery 9633 is shown as an example of a powergeneration means, there is no particular limitation on the powergeneration means and the battery 9635 may be charged with another meanssuch as a piezoelectric element or a thermoelectric conversion element(Peltier element). For example, the battery 9635 may be charged with anon-contact power transmission module which is capable of charging bytransmitting and receiving power by wireless (without contact), oranother charge means used in combination.

Further, an example of a moving object which is another example of theelectric devices will be described with reference to FIGS. 8A and 8B.

The lithium ion secondary battery described in any of the aboveembodiments can be used as a control battery. The control battery can beexternally charged by electric power supply using a plug-in technique orcontactless power feeding. Note that in the case where the moving objectis an electric railway vehicle, the electric railway vehicle can becharged by electric power supply from an overhead cable or a conductorrail.

FIGS. 8A and 8B illustrate an example of an electric vehicle. Anelectric vehicle 9700 is equipped with a lithium ion secondary battery9701. The output of the electric power of the lithium ion secondarybattery 9701 is adjusted by a control circuit 9702 so that the electricpower is supplied to a driving device 9703. The control circuit 9702 iscontrolled by a processing unit 9704 including a ROM, a RAM, a CPU, orthe like which is not illustrated.

The driving device 9703 includes a DC motor or an AC motor either aloneor in combination with an internal-combustion engine. The processingunit 9704 outputs a control signal to the control circuit 9702 based oninput data such as data of operation (e.g., acceleration, deceleration,or stop) by a driver or data during driving (e.g., data on an upgrade ora downgrade, or data on a load on a driving wheel) of the electricvehicle 9700. The control circuit 9702 adjusts the electric energysupplied from the lithium ion secondary battery 9701 in accordance withthe control signal of the processing unit 9704 to control the output ofthe driving device 9703. In the case where the AC motor is mounted,although not illustrated, an inverter which converts direct current intoalternate current is also incorporated.

Charge of the lithium ion secondary battery 9701 can be performed byexternal electric power supply using a plug-in technique. For example,the lithium ion secondary battery 9701 can be charged through a powerplug from a commercial power supply. The lithium ion secondary battery9701 can be charged by converting external power into DC constantvoltage having a predetermined voltage level through a converter such asan AC-DC converter. When the lithium ion secondary battery of anembodiment of the present invention is provided as the lithium ionsecondary battery 9701, shortened charging time or the like can beachieved and convenience can be improved. Moreover, the higher chargingand discharging rate of the lithium ion secondary battery 9701 cancontribute to greater acceleration and excellent performance of theelectric vehicle 9700. When the lithium ion secondary battery 9701itself can be made compact and lightweight with improved characteristicsof the lithium ion secondary battery 9701, the vehicle can be madelightweight and fuel efficiency can be increased.

It is needless to say that an embodiment of the present invention is notlimited to the electric devices illustrated in FIG. 6, FIGS. 7A to 7C,and FIGS. 8A and 8B as long as the electric devices are equipped withthe lithium ion secondary battery described in any of the aboveembodiments.

This application is based on Japanese Patent Application serial no.2011-282514 filed with Japan Patent Office on Dec. 23, 2011, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A method for charging a secondary battery, comprising:starting constant current charge; detecting a first battery temperaturebefore a voltage of the secondary battery reaches a given value;determining whether continuing the constant current charge in accordancewith the first battery temperature, wherein when the first batterytemperature is higher than or equal to T2, the constant current chargeis terminated; detecting a second battery temperature when the voltageof the secondary battery reaches the given value; and determiningwhether changing the constant current charge in accordance with thesecond battery temperature, wherein: when the second battery temperatureis lower than T1, the constant current charge is changed to constantvoltage charge, when the second battery temperature is higher than T1,the constant current charge is terminated, T1 and T2 are each giventemperatures, and T2 is higher than T1, the secondary battery comprisesa positive electrode including a positive electrode active materiallayer containing lithium iron phosphate, and the positive electrodeactive material layer is covered with single-layer graphene ormultilayer graphene including two to hundred layers.
 3. The methodaccording to claim 2, wherein T1 is higher than 40° C. and lower than orequal to 60° C.
 4. The method according to claim 2, wherein T2 is higherthan 60° C.
 5. The method according to claim 2, wherein the secondarybattery further comprises a negative electrode comprising a negativeelectrode active material layer containing graphite, and an electrolytecomprising a lithium salt and a solvent comprising ethylene carbonateand diethyl carbonate between the positive electrode and the negativeelectrode.
 6. A method for charging a secondary battery, comprising:starting constant current charge; detecting a first environmenttemperature after starting the constant current charge; determiningwhether continuing the constant current charge in accordance with thefirst environment temperature, wherein when the first environmenttemperature is higher than or equal to T2, the constant current chargeis terminated; detecting a second environment temperature when a voltageof the secondary battery reaches a given value; and determining whetherchanging the constant current charge in accordance with the secondenvironment temperature, wherein: when the second environmenttemperature is lower than T1, the constant current charge is changed toconstant voltage charge, when the second environment temperature ishigher than T1, the constant current charge is terminated, T1 and T2 areeach given temperatures, and T2 is higher than T1, the secondary batterycomprises a positive electrode including a positive electrode activematerial layer containing lithium iron phosphate, and the positiveelectrode active material layer is covered with single-layer graphene ormultilayer graphene including two to hundred layers.
 7. The methodaccording to claim 6, wherein T1 is higher than 40° C. and lower than orequal to 60° C.
 8. The method according to claim 6, wherein T2 is higherthan 60° C.
 9. The method according to claim 6, wherein the secondarybattery further comprises a negative electrode comprising a negativeelectrode active material layer containing graphite, and an electrolytecomprising a lithium salt and a solvent comprising ethylene carbonateand diethyl carbonate between the positive electrode and the negativeelectrode.